I'm not sure if this question properly belongs here or not; if it's in the wrong location then one of the moderators will move it to a more appropriate location.

Where a dual-gauge turntable is fitted on a conventional 3-rail system, how do traincrews turn the narrower-gauge loco? Would I be correct in thinking that the middle rail moves towards/away from the adjacent broader-gauge rail while the other broader-gauge rail separates to give 4 rails on the turntable lead?

The track on the turntable would have to be 4 track, i.e., the narrow gauge in the centre of the broader track. It has to be like that to keep the turntable properly balanced.
The narrow gauge track would have to be diverted from one side of the broader track to the centre of the broader track on the approach road or even earlier.
To do this would require a single switch similar to a catch point.
The track in the roundhouse or storage road could be be 4 track, as on the turntable or roads could be dedicated to either narrow gauge or the broader gauge.
Lining the turntable up would be the same, as the centre of the narrow gauge track is the same as the broader gauge.